1. Field of the Invention
The present invention relates to a control apparatus of an internal combustion engine.
2. Related Art
A car manufactured in recent years is provided with a catalyst such as a three-way catalyst, which is used for purifying exhausted gas, on the exhaust pipe of the car. At a cold start in which the engine is started at a low temperature of the engine and the catalyst, the ignition timing is retarded to increase the temperature of the exhausted gas. The increased temperature of the exhausted gas in turn promotes the heating of the catalyst so that the temperature of the catalyst is increased to a value in an active temperature range at an early time.
If the ignition timing is retarded in order to heat the catalyst at an early time, however, the engine torque decreases. In order to prevent the engine torque (or the engine speed) from decreasing, an idle speed control system (ISC) is used to increase the opening of an ISC valve (or a throttle valve) in order to raise an intake air quantity. As a result, the negative pressure of the intake air increases, reducing a difference between the negative pressure of the intake air and the atmospheric pressure. Accordingly, a braking force amplification effect of a brake booster inevitably becomes smaller.
In order to solve the above problems, as is disclosed in U.S. Pat. No. 5,497,745, with the initial value of the ignition timing at a cold start set at a target retard angle, ignition retarding control (or catalyst-early-heating control) is started and an intake manifold negative pressure is compared with a threshold value at predetermined control intervals. The threshold value is an intake manifold negative pressure required for assuring a proper negative pressure in a brake booster. If the intake manifold negative pressure is smaller than the threshold value, the ignition timing is retarded. If the intake manifold negative pressure is greater than the threshold value, on the other hand, the ignition timing is advanced.
As described above, with the technology disclosed in the USP, the initial value of the ignition timing at a cold start is set at a target retard angle and then the ignition timing is retarded or advanced in dependence on the intake manifold negative pressure. At a cold start, however, the fuel stability of the engine is poor so that, if the ignition timing is much retarded from the cold start as is the case with the disclosed technology, the fuel condition becomes unstable, unavoidably increasing the quantity of an exhausted unburned gas component such as HC or CO. In addition, if the ignition timing is much retarded from the cold start, the retard angle for the ignition timing causes a delay of the decreasing of the intake manifold negative pressure. Thus, it inevitably takes a longer time for the intake manifold negative pressure to decrease from a pre-start pressure (that is, the atmospheric pressure) to the threshold value, which is an intake manifold negative pressure required for assuring a proper brake booster negative pressure as described above. In the mean time, the negative pressure of the brake booster cannot be assured at a sufficient value so that the performance of the brake booster cannot be fully displayed. In short, with the disclosed technology, it is difficult to assure a sufficient negative pressure of the brake booster while reducing the exhaust emission at a start of the engine at the same time.
On the other hand, U.S. Pat. No. 3,129,802 discloses a technology whereby the closing timing of an intake valve is retarded when the pressure in a negative pressure tank for a brake booster is determined to be on the positive pressure side relative to a predetermined pressure. There is already known an apparatus (VVT) for adjusting a valve timing as is disclosed in JP-A No. S59-119007. The VVT is controlled to realize a valve timing proper for the operating state of the engine. The VVT is provided for achieving one of important objectives to improve the state of combustion. By execution of advancing control on the VVT in accordance with reduction of the negative pressure, however, the state of combustion cannot be improved sufficiently.
In addition, if the ignition timing is retarded in order to heat the catalyst at an early time, the resulting negative pressure is not sufficient as described above. Thus, with the technology disclosed in U.S. Pat. No. 3,129,802, advancing control is executed on the VVT in accordance with a negative pressure signal, resulting in an unimproved state of combustion.